Recently, there has been much development in the field of integrated circuit sensors utilizing silicon microstructures. In the prior art, silicon microstructures are made by etching a silicon substrate. Various homogenous and multiple layer cantilever beams and bridges have been made by selectively etching portions of the crystalline silicon that are not protected by an etch resistant layer. A major disadvantage and limitation of etching the silicon substrate is that such etching is not part of conventional integrated circuit planar fabrication technology. The etching step requires special handling during the fabrication of integrated circuits, such is generating a mask layer which may be difficult to control in alignment and tolerances with the standard planar fabrication masks. Furthermore, an anisotropic etchant for silicon, such as a mixture of ethylene diamine, pyrocatechol and water (EDP) or potassium hydroxide (KOH) is utilized to etch the silicon. Neither of the etchants are easily incorporated with conventional IC masking materials.
A known type of integrated circuit sensor useful for detecting the presence of a vapor utilizes a piezoelectric surface acoustic wave (SAW) device. In such a device, a mechanical wave is induced across the surface of the SAW device. A polymer film is disposed along the path of the wave. The polymer film is selected to sorb a gas of interest. In the presence of the gas, the sorption of the gas increases the mass of the polymer film. As the wave intercepts this film, its frequency, phase and time delay across the path of the film change. By detecting these changes, the presence of the vapor of interest is determined. A significant disadvantage and limitation of SAW devices is that they operate at very high frequencies, usually in excess of 10 MHz. At these frequencies, the design of the associated integrated circuit signal processing circuitry becomes more complicated, and hence, less economically feasible. A further disadvantage and limitation of SAW devices is that extra processing and fabrication steps are needed to integrate the piezoelectric with a silicon substrate in which the processing circuitry is fabricated.